Immunoassays utilizing antigen-antibody reactions have been extensively performed to determine slight amounts of substances in samples in the field of clinical examination. Among these, latex immunoturbidimetry with latex particles carrying antibodies (hereinafter also referred to as sensitized latex particles) has been extensively used in laboratories because the latex immunoturbidimetry can be achieved by a simple operation for a short time. In latex immunoturbidimetry, the amount of an antigen or an antibody in a sample is determined through optical detection of a change in absorbance caused by agglutination of sensitized latex particles during formation of immune complexes. This change in absorbance is based on an apparent change in particle size caused by agglutination of the sensitized latex particles.
As described in Patent Document 1, polystyrene latex particles mainly composed of polystyrene have been used in latex immunoturbidimetry because of ease in immobilization (sensitization) of antigens or antibodies specifically reactive with their target substances, relatively low cost, and easy control of the polymerization reaction of these particles. Regardless of such an advantage, i.e., physical adsorption (sensitization) of antigens or antibodies, the polystyrene latex particles can also adsorb non-target proteins in samples. This adsorption of non-target proteins may cause so-called non-specific reactions, i.e., agglutination reactions of sensitized latex particles not caused by a specific antigen-antibody reaction. The non-specific reactions should be suppressed.
According to Patent Document 1, latex particles sensitized with an antigen or an antibody are blocked with bovine serum albumin (BSA) to suppress the non-specific reactions. Unfortunately, such blocking is still insufficient, and generates high background values. Accordingly, this measure has a severe challenge for preparation of reagents enabling highly sensitive measurement.
Patent Document 2 discloses preparation of carrier particles for diagnostic agents. The particles are prepared by copolymerization of styrene, a compound represented by the formula: CH2═CR3—COO(CH2CH2O)x(CH(CH3)CH2O)y(CH2CH2O)zR4, where R3 represents H or CH3; R4 represents H or CH3; x, y, and z each represent 0 or an integer of 100 or less, and satisfy a relation 1≦x+y+z≦100, and a salt of styrenesulfonic acid in the presence of a water-soluble radical polymerization initiator in water. Patent Document 2 discloses use of only compounds represented by the formula where at least one of x, y, and z is 20 or more, and does not suggest use of any other compound. Furthermore, a high content of the compound represented by the formula is used in preparation of the polymer particles; hence, the resulting polymer particles barely adsorb antigens or antibodies physically onto their surfaces and cannot function as carrier particles for a diagnostic agent.
Patent Document 3 discloses latex particles suppressing non-specific adsorption prepared by co-immobilizing polyethylene glycols having different chain lengths and an antibody or an antigen onto the surfaces of the particles through covalent bonds to prevent desorption. Unfortunately, the latex particles should have a large number of functional groups for bonding antibodies and polyethylene glycol chains onto the surfaces of the particles, and should be prepared by a complicated process involving covalent bonding of antibodies, followed by covalent bonding of polyethylene glycol.